1. Field of the Invention
The present invention relates to a polyurethane foam. More specifically, the present invention relates to a polyurethane foam, a process for preparing the polyurethane foam, and its uses, for example, cushioning materials such as shoe soles.
2. Discussion of the Related Art
When a polyurethane foam is used as a cushioning material for shoe soles of sports shoes, its compression set as defined in ASTM D 395 is important. Therefore, it has been desired to meet the requirement of both compression set and strength. However, it is actually difficult to meet the above requirement For instance, if the crosslinking density of a polyurethane foam is increased in order to improve the strength, the compression set is, on the other hand, lowered.
Japanese Patent Laid-Open No. Hei 7-179556 discloses a process for preparing a thermoplastic polyurethane having a high hardness of not less than 75 (JIS-A hardness), excellent heat resistance, low compression permanent strain and excellent cold resistance, comprising reacting a polyester-polyol having a moleculer weight of 2500 to 3500 and 1,4-butanediol with a diisocyanate in a specific ratio. However, there arises some defects in this process that the thermoplastic polyurethane is poor in heat resistance, compression permanent strain and cold resistance, when the molded article density is adjusted to a desired range of 0.4 to 0.8 g/cm3, and Asker C hardness is adjusted to 50 to 75.
Japanese Patent Laid-Open No. Hei 7-179557 discloses a process for preparing a thermoplastic polyurethane which is excellent in moldability and has good heat resistance and cold resistance, comprising 1,9-nonanediol and 3-methyl-1,5-pentanediol as essential components for specific polyester diols. However, this polyurethane has some defects in that the polyurethane foam is poor in heat resistance, compression permanent strain and cold resistance, when the molded article density is adjusted to a desired range of 0.4 to 0.8 g/cm3, and Asker C hardness is adjusted to 50 to 75.
Also, Japanese Patent Laid-Open No. Hei 7-271263 discloses a process for preparing a cleaning blade comprising a thermoplastic polyurethane elastomer. This method is characterized in that heat treatment is carried out at a temperature of 60xc2x0 to 100xc2x0 C. for 8 to 16 hours to give a hardness of 50 to 80 (JIS-A). According to the method, a cleaning blade having a compression set (compression permanent strain) of not more than 30% is obtained. However, this cleaning blade has some defects in that the cleaning blade is poor in heat resistance, compression permanent strain and cold resistance, when the molded article density is adjusted to a desired range of 0.4 to 0.8 g/cm3, and Asker C hardness is adjusted to 50 to 75.
An object of the present invention is to provide a polyurethane foam having a high strength within a specific molded article density range and being excellent in compression set, which can be suitably used as cushioning materials such as shoe soles of sports shoes.
These and other objects of the present invention will be apparent from the following description.
According to the present invention, there is provided a process for preparing a polyurethane foam, comprising:
reacting a polyol having a number-average molecular weight of not less than 1000 and less than 2500 with a polyisocyanate compound in the presence of a catalyst and a blowing agent in a mold, to give a molded article having a density of 0.4 to 0.8 g/cm3, and
heating the resulting molded article to a temperature of 60xc2x0 to 100xc2x0 C.
Further, there are provided a polyurethane foam obtained by the above-described process and its applications.